24 chapter a closer look at arthropods - weebly

C H A P T E R

24 A Closer Look at Arthropods

KEY CONCE PTS

24.1 Arthropod DiversityArthropods are the most diverse of all animals.

24.2 CrustaceansCrustaceans are a diverse group of ancient arthropods.

24.3 ArachnidsArachnids include spiders and their relatives.

24.4 Insect AdaptationsInsects show an amazing range of adaptations.

24.5 Arthropods and HumansArthropods and humans interact in many ways.

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728 Unit 8: Animals

Connecting CONCEPTS

What is the relationship between these two insects?

Arthropod predators such as this digger wasp help to keep an important bal-

ance among Earth’s invertebrates. This digger wasp has captured a meal not for itself but for its young. The wasp will de-posit the live, but paralyzed, grasshopper into a burrow she has constructed. She will then lay a single egg next to the grass-hopper so when the egg hatches the larva will have a fresh meal.

Animal Behavior Unlike the solitary digger wasp, many insects live in large social colonies. This paper wasp nest has been carefully constructed of wood pulp mixed with the insects’ saliva. Within the colony, all insects are related to one another. A single queen lays eggs, which are raised by workers. Wasps will aggressively defend their nests from predators to ensure that the colony survives.

Chapter 24: A Closer Look at Arthropods 729

FIGURE 24.1 The hard exoskeleton of this rhino beetle is constructed of layers of chitin that help protect it from predators.

24.1 Arthropod DiversityKEY CONCEPT Arthropods are the most diverse of all animals.

MAIN IDEAS

• Arthropod features are highly adapted.

• Arthropod exoskeletons serve a variety of functions.

• Arthropod diversity evolved over millions of years.

VOCABULARY

arthropod,arthropod, p. 730

exoskeleton,exoskeleton, p. 730

chitin,chitin, p. 730

appendage,appendage, p. 730

segmentation,segmentation, p. 730

Reviewcuticle

Connect Earth is truly ruled by bug-eyed monsters. In just about every way,arthropods are the most successful animal phylum on Earth. More than three-fourths of all known animals—more than 1 million species —are arthropods.They play an important role in every ecosystem on the planet. What makes thisphylum so interesting? Arthropods are as diverse in shape and size as any lifeform on Earth, and are the result of millions of years of adaptation.

MAIN IDEA

Arthropod features are highly adapted.

Without knowing it, almost everywhere you go, you are interacting witharthropods. They can be found in the carpet you walk on and in the bed whereyou sleep. An arthropodarthropod is an invertebrate animal with an exoskeleton madeof chitin; a series of paired, jointed appendages; and segmented body parts.

Arthropod CharacteristicsThe entire surface of an arthropod’s body is covered by a protective exoskel-eton. An exoskeletonexoskeleton is an external skeleton that supports the animal’s tissuesagainst gravity. Arthropods, such as the rhino beetle in FIGURE 24.1, have exoskel-etons made of proteins and chitin. ChitinChitin (KYT-uhn) is a long organic moleculemade of sugars—similar to plant cellulose—that is arranged in layers. In eachlayer, fibers are laid out parallel to one another. But fibers in different layerspoint in different directions, forming a biological “plywood” that is very toughand strong. Like armor, chitin also protects the animal from predators.

Jointed appendages were an important adaptation during the evolution ofarthropods. An appendageappendage is an extension of an organism’s body. It can beused for walking, swimming, sensing, manipulating food, or chewing. Arthro-pods can have six, eight, ten, or even hundreds of appendages. The appendagescan be shaped like rakes, tweezers, nutcrackers, hammers, or paddles.

Arthropods have an incredible variety of body forms. Some are micro-scopic, while others are quite large. For example, some tropical stick insectsand millipedes can reach 30 centimeters (1 ft) in length, and spider crabs canhave an arm span of 3.6 meters (12 ft). But all arthropod bodies are segmentedfor specific functions. SegmentationSegmentation describes how an arthropod’s body partsare divided into similar sections that have each evolved for a different function.

730 Unit 8: Animals

FIGURE 24.3 Arthropod Diversity

Arthropods are a diverse group of animals. Millions of years of

evolution has led to many different body forms and functions.

CRUSTACEANS

Crab As aquatic arthropods, crabs have appendages adapted for both swimming and walking.

INSECTS

Butterfly Insects have three pairs of jointed appendages that are used for many dif-ferent functions.

MYRIAPODS

Centipede Many pairs of legs make centipedes suited to a wide variety of biomes.

CHELICERATES

Scorpion The carnivorous scorpions have sharp appendages for tearing apart their prey.

FIGURE 24.2 Trilobite fossils such as this one show the exoskeleton, segmentation, and jointed append-ages. These features led scientists to suggest that trilobites were one of the first marine arthropods.

TAKING NOTES

Use a main idea diagram to out-line the unique features of each arthropod group.

Trilobites

Crustaceans

Chelicerates

Arthropods

Insects

Myriapods

Arthropod GroupsClassifying animals with so many differences might seem difficult, but most fossil and living arthropod species can be placed into one of five groups.

• Trilobites Trilobites are now extinct but were an important part of Paleo-zoic marine ecosystems for nearly 300 million years. As you can see in FIGURE 24.2, their bodies were divided into three long vertical sections, or lobes. A central body and outgrowths of shell on each side covered their many delicate legs. Most of the 4000 known species were bottom feeders, sucking up muck, algae, or soft animals from the sea floor.

• Crustaceans Among the most familiar arthropods, crustaceans (kruh-STAY-shuhnz) are found in all of the oceans, freshwater streams, and even on land. Crustaceans are a diverse group that includes huge king crabs and lobsters, microscopic copepods, oysterlike barnacles, and armored pill bugs.

• Chelicerates The group known as chelicerates (kih-LIHS-uh-RAYTS)includes horseshoe crabs, scorpions, spiders, mites, ticks, and the extinct sea scorpions. These animals share a set of specialized daggerlike mouth-parts that are used for tearing their food.

• Insects Insects account for 80 percent of all known animal species. Familiar animals such as ants, bees, butterflies, moths, cockroaches, flies, and mosquitoes are all insects. Though this group is very diverse, most insects are terrestrial and have six legs.

• Myriapods The most commonly known myriapods (MIHR-ee-uh-PAHDS)are centipedes and millipedes. Their long bodies and many pairs of legs are the most distinctive characteristics of the myriapods. The largest species can grow up to a foot long. They generally live in humid environments, such as leaf litter, decaying wood, or moist soil. The first pair of legs in centipedes bear poisonous fangs for capturing prey.

Infer How did the evolution of jointed appendages lead to the wide variety of

arthropods we see today?


Apply What are the advantages and disadvantages of going

through the process of molting?

Watch a cicada molt at ClassZone.com.

BIOLOGY

FIGURE 24.4 Molting

Arthropods must molt their old

exoskeletons in order to grow.

32

1

ConnectingChemistry of Life Recall that in Chapter 2, you learned about the plant structure cellulose. The molecular structure of chitin forms a polymer similar to plant cellulose. This rigid structure contributes to the strength of an insect’s exoskeleton.

CONCEPTS

MAIN IDEA

Arthropod exoskeletons serve a variety of functions.

All arthropods have an exoskeleton and its structure determines how anarthropod lives. Recall that an exoskeleton is made of many layers of chitin.Chitin is not living tissue, and having a living body crammed into a hardexoskeleton is similar to how a medieval knight would wear a suit of armor.Despite the protective benefits, an exoskeleton makes important functions,such as movement, growth, and maintaining internal and external equilib-rium, difficult. Over millions of years, arthropods have developed ways ofmanaging normal functions that are both efficient and effective.

Movement and GrowthMovement Two types of cuticle plates assist in movement. Stiff cuticle platesof the exoskeleton are separated by sections of more flexible cuticle that formjoints in the hard armor. When muscles stretching across exoskeleton joints

contract, they bend the joint so the arthropodcan move. The cuticle supporting arthropodlegs acts as a spring, efficiently storing andreleasing energy as the animal moves.

Molting Arthropod cuticle cannot grow alongwith the animal, so an arthropod must shedits exoskeleton in a process called molting.This process of shedding and reforming a newexoskeleton is illustrated in FIGURE 24.4.

1 Before molting, the animal secretes anew layer of cuticle underneath itsexoskeleton. The new cuticle layer willactually be larger than the layer of cuticlecovering it.

2 The animal secretes enzymes that beginto digest and weaken the old cuticle,allowing the exoskeleton to split openand the animal to crawl out of it. This isas extreme as it sounds—the animalmust shed every surface of its body,including the entire lining of its gut andtracheae. Some animals die in the process.

3 The new exoskeleton is filled with fluidwhile it is still soft, making the animallarger than it was before the molt.However, the exoskeleton takes time toharden. During this time, the animal isvery vulnerable to predators.

Under the old exoskeleton, fluids are secreted that will form the new exoskeleton.

The insect sheds the old exo-skeleton in a process called ecdysis.

Once the arthropod crawls out of the old exoskeleton, the new exoskeleton will begin to harden.

732 Unit 8: Animals

Q U I C K L A B COM PAR I N G

Comparing ArthropodsIn this lab you will examine and compare the structures of different arthropods.

PROBLEM How do arthropods differ from one another?

PROCEDURE

1. Choose one of the four arthropod slides. Observe one organism under low power. Switch to high power and draw and label the structures of each organism.

2. Repeat the process with a second slide and compare features such as appendages, antennae, and body segments between the two organisms.

3. Find another group who viewed different specimens. View and note any differences between their specimens and yours.

ANALYZE AND CONCLUDE

1. Observe What unique structures did you notice on the slide specimens? Predict what uses these structures may have for the survival of each organism.

2. Evaluate What similar features do the organisms share? Based on your observations, how closely related do you think the specimens are?

MATERIALS• slide of tick• slide of mite• slide of spider• slide of mosquito• microscope

FIGURE 24.5 The compound eye of this fruit fly contains 800 indi-vidual eye units. These eyes are highly sensitive to movement and can also determine colors.

Managing Internal and External FunctionsCirculation Arthropods have an open circulatory system,in which blood is pumped through a tubelike heart and outinto the body cavity. In comparison, vertebrates have a closedcirculatory system, in which blood is contained inside a systemof arteries, veins, capillaries, and a heart. In an open circulatorysystem, blood is pumped through the heart and into the body,where it comes in direct contact with organs and tissues. Thestiff exoskeleton of arthropods also helps control blood pressure.Because an exoskeleton does not change shape, when the heartpumps blood out into the body, the exoskeleton keeps the bloodcontained, while body movements keep it circulating.

Senses Most arthropod sensory organs, including antennae, are madeof modified cuticle. Hard cuticle would otherwise block environmentalstimuli. Antennae and body hairs allow an arthropod to sense its surroundingenvironment, including temperature, touch, sound, and smell.

Most arthropods also have compound eyes. Unlike mammalian eyes, whichhave a single lens that collects all visual information, arthropod eyes havethousands of tiny individual lenses that interpret only a small portion of thefield of view. The image in FIGURE 24.5 gives an idea of how many individualeyes form a single compound eye. When all of these individual images cometogether, they form a rough mosaic of an object that resembles a newspaperor magazine image.

Summarize How does an exoskeleton make functions such as movement and

growth difficult?


24.1 ASSESSMENT

Connecting CONCEPTS

ONLINE QUIZClassZone.com

FIGURE 24.6 The velvet worm (top) and the water bear (below) belong to two phyla thought to be the closest relatives of arthropods. (colored SEM water bear; magnification 200�)

REVIEWING MAIN IDEAS

1. What are the five main groups of

arthropodsarthropods?

2. What characteristics make the

phylum Arthropoda unique?

3. Why are the relationships between

arthropod families so difficult to

determine?

CRITICAL THINKING

4. Contrast How are the structures

used for supporting organs different

in arthropods and humans?

5. Synthesize Fossils reveal that arthro-

pods have been walking the planet

for nearly 500 million years. How

have arthropods survived for so

long? What features have allowed

them to be so successful?

6. Anatomy In contrast to

arthropods, humans have an

internal skeleton. What

advantages and disadvantages

does this type of support

system have, as compared with

an exoskeletonexoskeleton?

MAIN IDEA

Arthropod diversity evolved over millions of years.

All of the major arthropod groups are incredibly old. The oldest arthropodfossils are trilobites from the early Cambrian period, about 540 million yearsago. The oldest known chelicerates and crustaceans appeared a few million yearslater, during the Cambrian explosion. The oldest known myriapod fossils areyounger—about 415 million years old—but trace fossils from rocks in Pennsyl-vania suggest myriapods are older than this. During the next 100 million years,rapid diversification of arthropod species resulted in the appearance of all themajor groups of arthropods. Many of today’s arthropods are very similar to thearthropods that lived hundreds of millions of years ago. Because the majorgroups of arthropods appeared so long ago, relationships between them aredifficult to determine, and many questions about classification still exist.

Based on similarities in body structures, some scientists think that arthro-pods are most closely related to annelid worms. Because both groups havesegmented bodies, some scientists hypothesize they share a similar ancestry.Recent molecular evidence indicates that annelids and arthropods may haveevolved segmentation independently. Two other members of the Ecdysozoa,velvet worms and water bears, are thought to be the closest living relatives ofthe arthropods. You can see these animals in FIGURE 24.6.

• Velvet worms (phylum Onchyophora) are soft-bodied carnivorous inverte-brates that can grow up to 10 centimeters in length and are covered with athin cuticle layer. They roam about the tropical forest floor on many un-jointed legs, hunting for termites and small mollusks.

• Water bears (phylum Tardigradia) are microscopic invertebrates, less thanone millimeter in length. Water bears are commonly found in the mud ofmarine, freshwater, and terrestrial environments. They are omnivores,feeding on plants, algae, dead organic matter, and other organisms.

Analyze Explain how ancient fossils can be used to determine relationships

between modern arthropods.

734 Unit 8: Animals

FIGURE 24.7 Crustaceans such as this violet-spotted reef lobster can be found in all of Earth’s oceans. They play an integral role in marine ecosystems.

24.2 CrustaceansKEY CONCEPT Crustaceans are a diverse group of ancient arthropods.

MAIN IDEAS

• Crustaceans evolved as marine arthropods.

• Crustacean appendages can take many forms.

• There are many different types of crustaceans.

VOCABULARY

crustacean,crustacean, p. 735

cephalothorax,cephalothorax, p. 735

abdomen,abdomen, p. 735

carapace, carapace, p. 735

mandible,mandible, p. 737

Reviewfilter feeding, sessile

Connect If you have ever eaten a shrimp, you may have an idea of what acrustacean looks like. But this large group of arthropods is surprisingly diverse.Shrimp and lobsters are crustaceans, but so are the tiny brine shrimp betterknown as “sea monkeys.” Tadpolelike copepods, oysterlike barnacles, and the pillbugs that live under damp rocks in your garden are also crustaceans.

MAIN IDEA

Crustaceans evolved as marine arthropods.

CrustaceansCrustaceans are a group of arthropods that have two distinct body sections,a hard exoskeleton, two pairs of antennae, and one pair of appendages persegment. Crustaceans evolved in the oceans, and today most crustaceans stilllive in saltwater environments. But there are also many freshwater species, anda few have evolved to survive on land.

Crustaceans come in a variety of shapes and sizes. They are vital to thestability of aquatic ecosystems. Some species, such as the violet-spotted reeflobster in FIGURE 24.7, are predators of marine fish, mollusks, and worms, whileothers scavenge dead animals and plants. Most importantly, crustaceans are asignificant food source for larger animals. Large crustaceans such as shrimp,lobsters, and crabs are a primary food source for fish, birds, seals, and evenhumans. But there are also numerous species of microscopic crustaceans.Zooplankton, krill, and copepods are very small as adults but are so abundantthat they constitute an important food source for fish, whales, and manyspecies of filter-feeding crustaceans.

Crustacean bodies are made up of two distinct body sections, acephalothorax and an abdomen. The cephalothorax cephalothorax (SEHF-uh-luh-THAWR-aks) is the region of an organism in which the head andtrunk region are combined into one long section. The abdomen abdomen

refers to the rear portion of the organism. The cephalothorax iscovered by a shieldlike section of cuticle called the carapace. Thecarapacecarapace (KAR-uh-PAYS) covers the sides of the body and protectsthe gills. The largest crustaceans have a cuticle layer with calciumdeposits that make the carapace a hard shell.

Connect Describe how a decrease in marine crustacean populations

could affect an ocean ecosystem.


FIGURE 24.8 Crustacean Anatomy

The anatomy of a crustacean is well suited to its underwater habitat.

Crustacean appendages are adapted for many different purposes.

FIGURE 24.9 The mantis shrimp is a crustacean that spends most of its time scavenging for food. Many species of shrimp and prawn inhabit the coastlines of every continent.

MOUTH AND MANDIBLES

mandible

cheliped

abdomen cephalothorax

swimmerets

walking legs

carapace

antennae

MAIN IDEA

Crustacean appendages can take many forms.

All crustacean appendages are homologous structures. Anatomical structuresare homologous when they share a common origin. For example, your arm ishomologous to a bat wing because long ago, humans and bats shared a com-mon ancestor. As the ancestors of bats and humans evolved, different uses forthe same appendage structures developed. In bats, the appendage evolved intoa winglike structure, while in humans, the appendage evolved into the armstructure we have today.

Long ago, crustacean appendages probably all looked the same. Today,crustacean appendages have been modified for a variety of different functions.Crustacean appendages are used for sensing the environment, defendingagainst predators, walking, feeding, and even attracting mates.

Crustacean appendages are adapted to an underwater habitat andare essential for survival. The illustration in FIGURE 24.8 shows manyof the different types of crustacean appendages. Probably the bestknown of crustacean appendages are the large claws that are foundon many species of crabs and lobsters. Lobsters’ claws, or chelipeds(KEE-luh-PEHDZ), are used for collecting and manipulating food.But chelipeds can be specialized for other jobs. Male fiddler crabshave large claws that are used to attract females. Some hermit crabsuse their claws to block the entrance to the shells in which they live.There are also some mantis shrimp, such as the one shown inFIGURE 24.9, that have hunting claws shaped like clubs. The clubs areattached to spring-loaded limbs that can strike at more than 22meters per second (50 mph), letting the shrimp break through snailshells to feed.

736 Unit 8: Animals

FIGURE 24.10 This coconut crab is the largest terrestrial arthropod in the world, and can have a leg span of almost 1 meter. Primarily a nocturnal creature, this arthropod feeds on coconuts it cracks open using a powerful cheliped.

TAKING NOTES

Use a main idea web to describe the many ways arthropods use jointed appendages.

Appendages

defense attracting mates

swimming walking

grasping food

All crustaceans have two pairs of antennae on their head. These append-ages have a covering of tiny hairs packed with chemical sensors. As antennae flick through the water, crustaceans use these appendages to smell food, locate mates, and avoid predators. Some crustaceans, such as the tiny water flea Daphnia, use the hairy antennae as oars to move around. Many species of spiny lobsters rub antennae against their cephalothorax to make loud calls that are thought to startle predators.

Crustaceans, like other arthropods, have mouths composed of a pair of hard appendages called mandibles. MandiblesMandibles are highly adapted appendages that crush and bite food before ingestion. Other appendages near the mouth can act as additional jaws and tear food into bits. In copepods, they can collect food from the water and push it toward the animal’s mouth.

Appendages such as walking legs allow a crustacean to move along the ocean floor as it scavenges for food. Swimming requires additional specialized appendages called swimmerets, which move in wavelike motions to propel the crustacean through the water.

As their appendages changed shape over time, crustaceans were able to use them for more than movement. This diversification of appendages allowed crustaceans to specialize and move into different ecological niches. The speciation of crustaceans through changes to their appendages is a good illustration of the process of natural selection.

Summarize Give three examples of crustacean appendages and how

they are used.

MAIN IDEA

There are many different types of crustaceans.

Crustaceans vary in both anatomy and structure. However, they are all similar in how they develop into the adult form.

DecapodsWhen you think of a crustacean, you are probably thinking of a decapod. This familiar group includes lobsters, crabs, and shrimp, as well as hermit crabs, their cousins the king crabs, and many others. Decapods live primarily in saltwater environments, but species such as crayfish have invaded fresh water. Some crabs, such as the coconut crab shown in FIGURE 24.10, even venture onto land to look for food.

Decapods have characteristic features that distinguish them from other crustaceans. The name decapod means “ten legs,” so all decapods have five pairs of jointed append-ages that are used for many different purposes. Decapods also have fused body segments that contain two regions, the cephalothorax and abdomen.


24.2 ASSESSMENT

Connecting CONCEPTS


FIGURE 24.11 Attached to the skin of this humpback whale, thousands of barnacles use the whale’s constant movement through the water to filter food from the water.

cement gland

mouth

legsstomach

Barnacles, Isopods, and Tongue Worms

Other species of crustaceans look nothing like a“typical” decapod. Although these species are differ-ent in structure from other crustaceans, scientistsdiscovered from developmental evidence that mostcrustaceans pass through similar phases as theygrow into adults. Almost all juvenile crustaceanspass through a free-swimming planktonic larval

stage. The most common of these larva forms is thenauplius larva. A nauplius larva has a carapace and

six long, feathery limbs. When scientists looked at newlyhatched barnacles and tongue worms, they found nauplius

larvae. Only crustaceans have nauplius larvae, so both groupsare recognized as crustaceans. But as they grow into their adult form,

these crustaceans look very different from any other crustacean.

• Barnacles Barnacles are sessile, or nonmoving, filter feeders wrapped ina heavily calcified shell. They live attached to the surface of rocks, boats,sea turtle shells, and even the skin of humpback whales, such as the oneshown in FIGURE 24.11. Barnacles have turned their carapace into a saclike“mantle” (similar to that found in a mollusk) that secretes a calcium shell.Inside the shell, the animal sits on its head and sweeps food into its mouthwith its legs.

• Isopods Isopods have flattened bodies and seven pairs of legs. Most speciesare marine and freshwater scavengers. Pill bugs and wood lice belong to agroup of isopods that have become completely terrestrial. These animalsusually live in damp habitats and eat rotting plants.

• Tongue worms Tongue worms are parasites that live in the lungs andnasal passages of vertebrates. They have no eyes, mandibles, or antennae,and they have lost most of their limbs. Despite this, molecular evidenceshows tongue worms to be most closely related to crustaceans.

Analyze What evidence helped scientists to classify barnacles and tongue worms

as crustaceans?


1. What characteristics make

crustaceanscrustaceans different from other

arthropods?

2. What are some of the different

functions of arthropod appendages?

CRITICAL THINKING

3. Summarize Draw a food web that

includes at least three different

types of arthropods.

4. Infer How did the discovery of

nauplius larvae allow scientists to

categorize barnacles and tongue

worms as crustaceans?

5. Evolution Lobster claws can

grow extremely large. What

selective pressures may have

led to the evolution of such

large appendages?

738 Unit 8: Animals

step 4

D E S I G N YO U R O W N I N V E S T I G AT I O NCHAPTER 24

Hatching Brine ShrimpBrine shrimp are small arthropods found in oceans, salt lakes, estuaries, and tidal

ponds. Brine shrimp are used primarily for fish food, especially on fish farms where

natural food sources are unavailable. Their eggs can be harvested and hatched

under controlled conditions by biologists working in an aquaculture facility. In this

investigation, you will design an experiment to determine the best conditions for

hatching brine shrimp eggs.

PROBLEM What are the best conditions for hatching brine shrimp eggs?

PROCEDURE

1. Choose one factor to investigate. Some factors to consider are the presence or absence of light, aeration (open container versus air-tight container), and pH (6–8). Make a hypothesis about that factor, and design an experiment to test your hypothesis. Have your teacher approve your design.

2. The conditions of the hatching (temperature, pH, light conditions, and so on) will depend upon your independent variable. If you are testing pH, use vinegar to adjust the pH of the sea salt solution to pH 6.0 or baking soda to adjust the pH to 8.0 as measured with the pH test paper.

3. Determine the number of cups of brine shrimp needed based on your experimental design. Be sure to include a control. Hatch the brine shrimp eggs by filling each cup with 200 mL sea salt solution and 0.1 g brine shrimp eggs. Cover the cups with plastic wrap or aluminum foil depending upon your experiment. If you are testing aeration, use bottles with lids instead of cups.

4. Once you have set up your cups or bottles, remove 5 mL from each with an eyedropper and place each volume in a separate clean petri dish.

5. Examine each dish with a hand lens or microscope. Count the number of eggs and the number of hatched brine shrimp. Note these numbers.

6. Repeat steps 4 and 5 each day for five days and note the numbers of hatched shrimp and eggs each day.


1. Analyze Graph the data with the number of shrimp hatching or number of shrimp hatching rate (shrimp hatching divided by number of eggs) on the y-axis and the days on the x-axis.

2. Analyze On what day did you observe the most shrimp hatching? Was this day consistent with the findings of other members of your group? List possible reasons for inconsistent results.

3. Experimental Design Identify possible sources of error in your experiment and give reasons why they might have occurred.

4. Analyze How did your factor affect the hatching of brine shrimp?

5. Infer Compare your team’s result with other teams in your group. Identify conditions that had the most positive effect on the hatching of brine shrimp.

6. Experimental Design Explain why it was important to have a control in your experiment.

MATERIALS• clear plastic cups• 100-mL graduated

cylinder• 400 mL sea salt solution• 0.1 g brine shrimp eggs• plastic spoon• balance• clear plastic wrap• aluminum foil• clear plastic bottles

with lids • vinegar• baking soda• pH duo-test paper• 2 eyedroppers • 10 petri dishes• hand lens or dissecting

microscope• lamp

PROCESS SKILLS• Designing Experiments• Collecting Data• Analyzing• Predicting


FIGURE 24.12 The bright color-ation of the spiny spider actually attracts insects to the spider’s web, where they are trapped and made into an easy meal.

24.3 ArachnidsKEY CONCEPT Arachnids include spiders and their relatives.

MAIN IDEAS

• Arachnids are the largest group of chelicerates.

• Arachnids have evolved into a diverse group.

VOCABULARY

chelicerate,chelicerate, p. 740

arachnid,arachnid, p. 740

book lung,book lung, p. 740

spiracle,spiracle, p. 741

trachea,trachea, p. 741

Connect At first, the bump on Emily’s back looked like an insect bite. But soonthe rash grew into a bulls-eye that was six inches wide. Emily began complainingof aches, and her temperature shot up to 102 degrees. A trip to the doctor gaveher parents a startling surprise —Emily had Lyme disease. This disease is causedby bacteria that is carried and spread by a tiny arachnid called a deer tick.

MAIN IDEA

Arachnids are the largest group of chelicerates.

Deer ticks are chelicerates—arthropods without mandibles. CheliceratesChelicerates (kih-LIHS-ur-AYTS) are arthropods that lack antennae and have six pairs of append-ages, which include four pairs of walking legs. One set of highly modifiedappendages form fanglike mouthparts called chelicerae, which are used tomash up food and shove it into a holelike mouth. A second set of appendages,called pedipalps, are used to grasp and subdue prey. Chelicerate bodies havetwo sections: a cephalothorax and an abdomen.

There are three main groups of chelicerates. The horseshoe crabs and seaspiders are two of these groups. Arachnids are the third group, representingmore than 80 percent of all chelicerate species. Arachnids,Arachnids, such as the spinyspider shown in FIGURE 24.12, are a terrestrial group of chelicerates character-ized by eight legs, fanglike pincers that inject venom, and the ability toproduce silk.

Evidence from fossils nearly 400 million years old suggests that arachnidsevolved adaptations that allowed them to conserve water and live on land.Arachnids have four different adaptations that reduce water loss.

• Waterproof cuticle An arachnid’s cuticle is waterproof, so water cannotevaporate across the skin.

• Book lungs Some arachnids have specialized respiratory structures calledbook lungs that allow them to breathe air. Book lungs Book lungs are structures builtof many thin, hollow sheets of tissue that look like the pages of a book.They provide a large surface for gas exchange but also create a very largesurface for water loss. To prevent water loss, book lungs are enclosed in ahumid chamber covered by a plate of abdominal cuticle.

• Malpighian tubules Excretory structures called Malpighian (mal-PIHG-ee-uhn) tubules allow spiders to minimize loss of water while excretingmetabolic wastes.

740 Unit 8: Animals

FIGURE 24.13 Arachnid Anatomy

Arachnids have many unique adaptations for catching

and consuming their prey.abdomencephalothorax

eyes

spinnerets

brain

poison gland

heart

book lung

spiracle

ConnectingNeurotransmitters Neurotoxins in the venom of spiders disrupt neurotransmitter function and lead to paralysis of a victim. You will learn more about the ner-vous system and neurotransmit-ters in Chapter 29.

CONCEPTS

VOCABULARY

The word spiracle comes from the Latin word spirare meaning “to breathe.”

• Spiracles Some species of arachnid use spiracles to breathe. Spiracles Spiracles aretiny holes on the abdomen that open and close to allow oxygen to enter.Oxygen flows through a series of tubes called tracheae. TracheaeTracheae (singular,trachea) carry oxygen directly to the arachnid’s tissues.

Explain How do the features of an arachnid allow it to live on land?

MAIN IDEA

Arachnids have evolved into a diverse group.

Spiders make up half of the more than 60,000 known arachnid species. Theyare predators that hunt or trap their prey. All spiders have the ability to makesilk and produce venom. Silk is made by glands in the spider’s abdomen andreleased by modified appendages called spinnerets. Spiders use silk for build-ing webs, wrapping prey and egg cases, building shelters, and producing draglines that anchor the animal like a climber’s safety line.

All spiders produce venom. Spiders inject venom into their prey throughmodified chelicerae. Neurotoxic proteins paralyze the victim by attacking thecentral nervous system, and digestive enzymes in the venom begin to dissolvethe prey from the inside. Among North American spiders, only the black widowand brown recluse spiders have venom that can affect a human, but manyspecies around the world have venom that can paralyze or even kill humans.

Mites, ticks, and chiggers are a large group of very small arachnids. Somespecies are less than 0.25 millimeters long. Many are parasites of plants andanimals, sucking up sap or blood through their needlelike mouthparts.Scorpions are arachnids with huge pincers for grabbing their prey after ithas been injected with venom from a stinger at the end of their tail.Scorpions hunt at night by feeling the vibrations made as their prey moves.


24.3 ASSESSMENT

Connecting CONCEPTS


D A T A A N A LY S I S

CONSTRUCTING SCATTERPLOTSSpiders produce silk for a wide variety of purposes. Some spiders build webs to capture prey, or to safely wrap their eggs. It is important that silk be strong enough to support the spider as it moves from one place to another. Scientists studying spider silk measured the diameter of silk strands in relationship to the body length of the spider. Their results are summarized in the table below.

1. Graph Data Construct a scatterplot using the data from the table.

Be sure to label the axes and to include a title for your graph. Refer

to page 721 in Chapter 23 on how a scatterplot is constructed.

2. Interpret What is the relationship between silk diameter and

spider length?

3. Interpret Does your scatterplot data show any outliers? How

might a larger data set help in interpreting the validity of outliers?

TABLE 1. SPIDER LENGTH AND SILK DIAMETER

Body length (mm) 7 8 10 11 12 15 16 17 18 20 25 26 26 35 40 45

Silk diameter (mm) .02 .03 .03 .04 .04 .06 .05 .08 .06 .05 .08 .09 .10 .18 .14 .15

To find out more about arachnids, go to scilinks.org.

Keycode: MLB024


1. What unique features do all

arachnidsarachnids share?

2. What four adaptations do arachnids

have for conserving water?

3. Why are spiders such an important

part of an ecosystem?

CRITICAL THINKING

4. Summarize How do bookbook lungslungs

help arachnids to reduce water loss?

5. Infer Spiders use different kinds of

silk for different purposes. What

might a spider build with sticky silk?

6. Ecology Describe a situation in

which spiders would be a

secondary consumer. What

species might prey on a spider?

Draw a simple food web to

illustrate your answer.

Most scorpions eat insects, spiders, and other scorpions, while some of thelargest species eat lizards and small rodents.

Arachnids are important prey species for vertebrates, but they are evenmore important as predators. Spiders are some of the most widespreadpredators on the planet and play an important role in most terrestrial foodwebs. The mass of the insects they eat each year is larger than the combinedmass of all human beings. Mites and ticks also have a major impact on ecosys-tems. Spider mites are serious pests of fruit trees, cotton, and other crops.Ticks can transmit serious human diseases such as Rocky Mountain spottedfever and Lyme disease.

Infer How might the loss of many arachnid species affect an ecosystem?

742 Unit 8: Animals

FIGURE 24.14 This potter wasp illustrates the three-part body structure of all insects: head, thorax, and abdomen.

headthorax

abdomen

24.4 Insect AdaptationsKEY CONCEPTS Insects show an amazing range of adaptations.

MAIN IDEAS

• Insects are the dominant terrestrial arthropods.

• Insects undergo metamorphosis.

• Insects have adapted to life on land.

VOCABULARY

incomplete incomplete metamorphosis,metamorphosis, p. 744

completecompletemetamorphosis,metamorphosis, p. 744

pupa,pupa, p. 744

Connect Everywhere you turn, there are more of them. They are under yourfeet, flying over your head, nestled in your clothing fibers, and waiting for you togo to bed. Insects are virtually everywhere, and many times you may not evenknow it. With more than 900,000 known species, insects are the single largest andmost diverse group of animals on the planet.

MAIN IDEA

Insects are the dominant terrestrial arthropods.

Insects are an incredible success story. Like the arachnids, they invaded landaround 400 million years ago. They have many of the same adaptations forterrestrial life as arachnids do. Insects have moved into virtually every ecologi-cal niche, which has helped them diversify into the largest group of animals.

Insects can be found in the most extreme places, including hot sulphursprings and the soil of Antarctica. They are also found in streams and ponds.Though some species live in the marine intertidal zone, they are largely absentin the seas, where crustaceans are the dominant arthropods. Due to such awide distribution, scientists are discovering new species of insects each day.

All insects have a body with three parts: a head, a central region called thethorax (THAWR-AKS), and an abdomen, as shown in FIGURE 24.14. The thoraxhas three pairs of legs, and most adult insects also have two pairs of wings.

Insects usually have one pair of antennae and onepair of compound eyes. Many have mandibles thatthey use to chew up their food, but others havemodified mouthparts for more specialized feedingbehaviors.

Some insect species live in colonies of hundredsor thousands of individuals. Within these colonies,complex social structures exist. Individuals performspecific jobs that help the colony be successful.Scientists believe that an insect’s genetic code deter-mines what role it will play in a colony.

Compare and Contrast How are insects similar to and

different from crustaceans and arachnids?


FIGURE 24.15 In the process of incomplete metamorphosis, a nymph appears to be a miniature copy of the adult insect. But on closer inspection, these immature insects lack important features such as wings and sex organs.

egg

nymph stages

VOCABULARY

The word metamorphosis comes from the Greek word metamorphoun, which means “to transform.”

ConnectingPlants Recall from Chapter 21 that stomata are holes in the leaves of plants that open and close to control transpiration and gas exchange. Many arthropods use spiracles in the same way, opening and closing them to control water and gas exchange.

CONCEPTS

MAIN IDEA

Insects undergo metamorphosis.

Insects do not develop the same way you do. When you were first born, youhad all of the same body parts as an adult. The same is not true of mostinsects. Some insects, such as butterflies and mosquitoes, go through dramaticphysical changes between their immature and mature forms.

Some insects, such as grasshoppers and cock-roaches, look like miniature adults when they hatch.This pattern of development, illustrated in FIGURE 24.15,is incomplete metamorphosis incomplete metamorphosis (MEHT-uh-MAWR-fuh-sihs), or direct development. These immature insectsare often called nymphs. They have six legs and ahead, thorax, and abdomen, but they do not havewings or sex organs. Nymphs get larger with eachmolt, but only grow wings and sexual organs duringthe later molting stages.

In the process of complete metamorphosis complete metamorphosis,illustrated in FIGURE 24.16, young insects do not looklike adults but molt and change their form as theymature. Young insects hatch out of eggs as wormlikelarvae whose bodies are not clearly divided into a

head, thorax, and abdomen, and they often lack legs or antennae. As theygrow, larvae pass through several molts, getting bigger each time, until theymolt into an inactive form called a pupa.pupa. Inside the pupa, some tissues arebroken down for energy and others are reorganized to produce a completelynew body form. When the adult insect emerges from the pupa, it looks verydifferent from a larva. It has wings, legs, and compound eyes, and is ready tofly away and begin its search for a mate.

Connect Which type of metamorphosis more closely resembles human

development? Explain.

MAIN IDEA

Insects have adapted to life on land.

Insects have several adaptations that allow them to be successful terrestrialspecies. Just like arachnids, insects retain water for survival using exoskeletons,Malpighian tubules, spiracles, and tracheae. Gases move through the tracheaeby diffusion, and spiracles can close to prevent water loss. Some insects canalso pump air through their bodies by rapidly squeezing and expanding thetracheae. The water-conserving characteristics that are shared by insectsand arachnids are an example of convergent evolution. In both groups,individuals that could survive longer without water were better able to repro-duce. In this way, natural selection independently moved both groups towardsimilar features that allowed them to carry out similar water-conservingfunctions.

744 Unit 8: Animals

brain

heart

thoracic legs

silk gland

antenna

proboscis

stomach

rectum

heart

spiracles

See a butterfl y go through metamorphosis at ClassZone.com.

BIOLOGY

CRITICAL

VIEWING

During which phase of development do you think the monarch butterfly is most

vulnerable to predators?

FIGURE 24.16 Complete Metamorphosis of a Monarch Butterfly

The complete metamorphosis of an insect involves two completely different

forms: a juvenile form and an adult form. This prevents adults and juveniles

from competing for the same resources.

1

4

2Pupa The caterpillar sheds its skin one final time and forms a shell called a chrysalis. Inside, dramatic changes trans-form the pupa.

Eggs Female butter-flies deposit eggs on the underside of a leaf. Three to six days later, the small eggs hatch into larvae.

Larva A butterfly larva is also called a caterpillar. After hatch-ing, caterpillars spend most of their time feeding and also molt their skin as they grow larger.

3Adult The monarch butterfly that emerges from the chrysalis 9 to 14 days later is very different from the caterpillar larva. The butterfly is now an adult and begins the life cycle again.


24.4 ASSESSMENT

Connecting CONCEPTS


FIGURE 24.17 Bees can beat their wings 200 times per second. This ability allows them to hover in one place while looking for food.

TAKING NOTES

Use a Venn diagram to catego-rize insects based on their mouth parts.

bothchewing sucking

Only four groups of animals have evolved true flight: insects,the extinct pterosaurs, bats, and birds. Insects conquered the airfirst and have been flying for nearly 400 million years.

Insect wings are long, flat extensions of the exoskeleton thatstick out of the animal’s back. At the base of each wing aremuscles that move the wing up and down, allowing insects, suchas the bee in FIGURE 24.17, to fly. But it takes a great deal of energyto move a wing up and down. The insect exoskeleton has evolvedin ways that conserve the energy needed for flight. By enablingthe insect to reuse the stored kinetic energy created by wingmovement, muscles attached to wings do only part of the work.For example, when a diver uses a springboard, the diver’s masscreates kinetic energy as the board flexes. When the boardrebounds, this energy is released and the diver is shot into the air.Like a springboard, insect cuticle is slightly flexible. As the wing

moves, it exerts force on the exoskeleton, causing it to flex and slightly deform.When the cuticle rebounds, the stored kinetic energy moves the wing back toits original position. In this way, an insect reduces the amount of energyneeded for flying, leaving more energy available for feeding and reproduction.

Insects feed on many different foods. Over time, insect mouth shapes havetaken on a variety of forms according to their diets. Generally speaking, insectseat by either chewing or sucking. Insects such as butterflies and moths thatfeed on plant nectar and fruits have a long, strawlike mouth called a proboscis(proh-BAHS-sihs). A mosquito also has a proboscis, but instead it uses itsmouth to suck blood from other animals. Other insects feed by chewing theirfood. Ants, beetles, and dragonflies have chewing mouthparts, which consistof mandibles adapted either to crushing leaves and plant stems, or to tearingapart flesh. Bees, wasps, and some flies are insects that have adapted to feed byboth sucking and chewing.

Contrast How might the chewing mouthparts of carnivorous and herbivorous

insects be different?


1. What features make insects differ-

ent from other arthropods?

2. What is the difference between

complete metamorphosiscomplete metamorphosis and

incompleteincomplete metamorphosismetamorphosis?

3. What are two major adaptations

that helped insects to survive

on land?

CRITICAL THINKING

4. Hypothesize How might the

spiracles of a desert insect be

different from the spiracles of a

tropical rain forest insect?

5. Infer Give two reasons why flight

has enabled insects to be so

successful.

6. Evolution Hundreds of

millions of years ago, spiders

began to use silk around the

same time insects began to fly.

Explain how natural selection

affected these changes.

746 Unit 8: Animals

FIGURE 24.18 Aphids devour plant tissues, often killing the host plant. Arthropod pest spe-cies can cause damage to crops, forests, and even homes.

24.5 Arthropods and HumansKEY CONCEPT Arthropods and humans interact in many ways.

MAIN IDEAS

• Arthropods and humans share many of the same resources.

• Some arthropods can spread human diseases.

VOCABULARY

insecticide,insecticide, p. 747

vector,vector, p. 748

Reviewbiomagnification

Connect People have a love-hate relationship with arthropods. Some arthropodsare important pollinators that fertilize human crops. Others are pests thatdestroy crops and infest our homes. Still others are predators that eat pests.Insect species are used for food and fibers in many human cultures, but otherinsect species spread human diseases. Over time, the unavoidable interactionsbetween arthropods and humans have created many conflicts.

MAIN IDEA

Arthropods and humans share many of the same resources.

Many arthropods are herbivores, and many of them eat the same plants peopleuse for food, textiles, and building materials. These arthropods compete withhumans for the same resources. Competition is stiff, because there are farmore arthropods than people. For example, when you look under the leaves ofapple trees or pepper plants, you may see clusters of tiny bumps. These bumpsare insects called aphids. Aphids, shown in FIGURE 24.18, use their needlelike

mouthparts to pierce the cell walls of a plant and suck up the sugaryliquid inside. A single aphid is small and cannot do much damage on itsown. But aphids live in large colonies. Hundreds of aphids on a plantcan remove enough sap to damage or kill the plant.

Each year, arthropods cause millions of dollars in damage to cropssuch as corn, wheat, and cotton. To prevent costly infestations, farmersuse insecticides to control arthropod populations. An insecticideinsecticide is achemical compound that kills insects and other arthropods. But spray-ing toxic chemicals on plants can have unwanted side effects. Manyinsecticides are toxic to other animals, including people. Some, such aschlordane and DDT, do not break down quickly. They can accumulatein predator species through the process of biomagnification. Arthro-pods can also become resistant to insecticides through natural selection.This resistance causes humans to use even larger doses of the toxin.

To avoid the potential hazards of using insecticides, scientists havediscovered ways to use the unique characteristics of arthropods to findsafer ways of controlling pest populations.


FIGURE 24.19 Many arthropods are important pollinators. In addi-tion to getting a nectar meal, this Eastern tiger swallowtail butterfly helps to pollinate flowers by car-rying pollen from one flower to another.

TAKING NOTES

Use a main idea diagram to list the diseases carried by arthropod vectors.

Arthropod Vector Diseases

Bubonic Plague

• Insecticides can be developed to be specific to arthropods. One example is a neurotoxin that blocks a particular receptor that is common in arthropod nerves but not found in other animal nerves.

• Integrated pest management, or IPM, reduces the number of insect pests on a plant crop by managing their ecology. By using a variety of other methods including insect traps, physical barriers, and introduced predators such as ladybugs and parasitic wasps, IPM helps to control pest species.

• Genetically modified plants can be made to resist particular pest species. For example, Bt corn has been engineered to include a gene from a soil bacterium, Bacillus thuringiensis, artificially inserted into its DNA. The gene makes a protein that kills caterpillar pests such as the European corn borer but is harmless to most other animals.

Infer What would be a disadvantage to introducing a predator insect for pest

management?

MAIN IDEA

Some arthropods can spread human diseases.

Humans are a source of food to arthropods such as mosquitoes, biting flies, fleas, and ticks. FIGURE 24.20 shows how arthropods can also be vectors that carry diseases. A vector vector is an organism that carries a disease from one host to another.

Diseases spread by arthropods can have serious effects on human popula-tions. Many methods for controlling arthropod vectors have been developed. Vaccinations have been developed to protect individuals from many types of diseases by delivering small doses of the pathogen to the immune system, which can then fight off future pathogen invasions. The use of pesticides targeted to specific arthropods can also help slow the spread of disease.

• Bubonic plague is caused by a bacterium carried by a flea. The disease normally affects rodents such as prairie dogs, squirrels, and rats. Human infections occur when a flea that has fed on an infected rat feeds on a human. Outbreaks of bubonic plague devastated European cities between the 1300s and the 1600s. The largest of these epidemics, between 1347 and 1350, killed between one-third and one-half of the people living in Europe. Today, bubonic plague is controlled by antibiotics and improved hygiene.

• Yellow fever is caused by a virus and normally affects monkeys, but it can be carried to humans by mosquitoes. The virus causes fever and bleeding. It was common in the United States until the early part of the 1900s and is still common in Africa and South America. Yellow fever epidemics killed nearly 20,000 people during the construction of the Panama canal before mosquito eradication programs brought the disease under control.

• Malaria is caused by a protozoan parasite carried by mosquitoes. The para-sites enter red blood cells to breed, periodically emerging and destroying them. Like yellow fever, malaria was once common in the United States and Europe. Malaria was largely eliminated in temperate countries during the 1950s by a program of DDT spraying. It is still common in tropical regions of Africa, Asia, and Central and South America.

748 Unit 8: Animals

24.5 ASSESSMENT

Connecting CONCEPTS


Arthropods are vectors for carrying diseases such as Lyme disease.

Analyze Though vectors can bring disease into human populations, explain why diseases carried

by arthropods spread more quickly through the populations of other animals.

FIGURE 24.20 Arthropod Vectors

2 31As the tick feeds on the host’s blood, microscopic pathogens pass from deer to tick. (colored SEM tick; magnification 15�)

Inside the tick, the pathogens are stable and do not affect the tick. (LM; magnification 4000�)

When the tick feeds on another host, pathogens are passed from the tick to the new host.

ConnectingImmune System Vaccines can only work to prevent a disease; they cannot cure a person who is already sick. The human immune system has specifically designed cells which work to fight diseases in your body.

CONCEPTS• West Nile virus is contracted from mosquito carriers that have previouslyfed on birds infected with the virus. Originally discovered in Africa, WestAsia, and the Middle East, the virus has spread throughout the world andhas been identified in 46 of the United States. Fever, headache, and skinrashes are some of the minor symptoms, but the virus can lead to meningi-tis or encephalitis, potentially fatal diseases. Scientists are currently work-ing to control the spread of the virus.

Infer Explain how an organism other than an arthropod could be a vector.


1. What are three effective ways of

managing insect pest populations?

2. How does a vector vector spread a disease

such as malaria?

CRITICAL THINKING

3. Analyze What are the potential

costs and benefits of using an

introduced predator to control a

pest population?

4. Connect What effect, if any, would

the development of a vaccine

against a tick-borne disease have on

the tick population? Explain.

5. Natural Selection Describe a

situation in which natural

selection could lead to an

arthropod population that is

resistant to pesticides.


MATERIALS• cotton swab• tissues• petroleum jelly• microscope slide• culture of Daphnia magna• 2 eyedroppers• cover slip• stopwatch• microscope• hydrogen peroxide solutions

heart

CHAPTER 24 O P T I O N S F O R I N Q U I RY

Use these inquiry-based labs and online activities to deepen yourunderstanding of arthropods.

Daphnia and Heart RateThe water flea, or Daphnia, is a tiny crustacean that

lives in fresh water. Biologists often use Daphnia to

study the effects of various chemicals, such as

hydrogen peroxide, on freshwater ecosystems.

SKILLS Collecting Data, Graphing, Analyzing

PROBLEM How does hydrogen peroxide affect Daphnia heart rate?

PROCEDURE

1. Make a ring of petroleum jelly on a slide and make wet mount with a Daphnia sample.

2. Observe the Daphnia under low and high power.

3. Looking through the microscope, measure the heart rate of the Daphnia by tapping a pencil on the desk once for each heartbeat. Your partner should count the number of taps in 15 seconds. Calculate the number of beats per minute by multiplying this number by 4. Take the average of three trials, and record the data in a data table.

4. Predict the effect of adding hydrogen peroxide to the Daphnia environment, and record it in your notebook.

5. Place two drops of one of the hydrogen peroxide solutions on one side of the wet mount. Draw the liquid under the cover slip by placing the edge of a tissue on the opposite edge of the cover slip.

6. Repeat step 3 to measure the Daphnia heart rate.

7. Calculate the change in heart rate by subtracting the average heart rate observed after adding hydrogen peroxide from the average heart rate observed in water.

8. Collect average heart rate data from other groups who used different hydrogen peroxide concentrations.


1. Analyze Using your own data and the data collected by your classmates using different hydrogen peroxide concentrations, construct a graph with the change in heart rate on the y-axis versus hydrogen peroxide concentration on the x-axis.

2. Interpret How did hydrogen peroxide affect the heart rate of Daphnia? At what concentration was the effect greatest?

3. Experimental Design What are possible reasons for error in the experimental design?

I N V E S T I G AT I O N

Daphnia

750 Unit 8: Animals

BIOLOGY

CLASSZONE .COM

A N I M AT E D B I O L O G YWhat Type of Arthropod?Can you tell what group an arthropod belongs to just by looking at it? Examine a series of arthropod images and try to categorize them. Be careful—looks can be very deceiving!

W E B Q U E STArthropods live in almost any environment. As a class, develop a field guide of arthropods in your area. Research and report on each arthropod’s habitat, feeding habits, life cycle and evolutionary history.

V I RT UA L L A BInsects and Crime Scene AnalysisHow can bugs help solve a crime? In this interactive lab, you will examine insects found on a corpse to determine how long the body was lying in a field.

I N V E S T I G AT I O N

Inside a Crayfi shA crayfish is a freshwater crustacean similar to

shrimp, crabs, and lobsters. In this lab, you will

dissect and examine the parts of a crayfish.

SKILL Observing

PROBLEM How do the form and function of crayfish organs help it survive in a marine environment?

MATERIALS• dissecting tray• scissors• forceps• dissecting needle• 12 dissecting pins • preserved crayfi sh specimen• hand lens or dissecting microscope• paper towels• paper and pencil• Anatomical Crayfi sh Drawings

PROCEDURE

1. Examine the external anatomy of the crayfish. Draw and label a picture of the crayfish anatomy, using illustrations in your handout. Label the structures listed on the drawing.

2. Turn the crayfish on its side and remove the legs below the carapace.

3. Using the forceps and scissors, lift and cut the carapace to expose the featherlike gills.

4. Carefully remove the gills and the joints. Cut the remaining plates from the top (dorsal) midline to the base of each leg to expose the internal organs. Examine the inside of the crayfish and identify as many internal organs as you can.


1 . Infer What do you think the antennae and antennules do?

2. Analyze How does the feathery structure of the gills help with their function?

3. Infer Why do you think the gills are attached to the legs?


24CHAPTER

@ CLASSZONE .COM

KEY CONCEPTS Vocabulary Games Concept Maps Animated Biology Online Quiz

carapace

cephalothoraxabdomen

24.1 Arthropod DiversityArthropods are the most diverse of all animals.

The five major groups of arthropods are trilo-

bites, crustaceans, chelicerates, insects, and myr-

iapods. Each group has unique features that have

evolved over mil-

lions of years, but

all share features —

including an exo-

skeleton made of

chitin, and jointed

appendages.

24.2 CrustaceansCrustaceans are a diverse group of ancient

arthropods. Most crustaceans are aquatic

arthropods with segmented bodies, a hard exo-

skeleton, two pairs of antennae, and one pair of

appendages per segment that set crustaceans

apart from other arthropods. The recognizable

decapods have two major body segments: a

cephalothorax and an abdomen. Isopods, barna-

cles, and tongue worms appear very different but

share the same characteristic features. The

appendages of crustaceans are highly adapted to

each species’ habitat and niche.

24.3 ArachnidsArachnids include spiders and their

relatives. Chelicerates are arthropods

that are distinguished by four pairs of

walking appendages and two pairs

of modified appendages used for

feeding. The arachnids are terres-

trial chelicerates that have evolved book

lungs and other adaptations for survival on land.

The most common arachnids are spiders, but

mites, ticks, chiggers and scorpions are also mem-

bers of this family. They play an important eco-

logical role as invertebrate predators.

24.4 Insect AdaptationsInsects show an amazing range of adaptations.

Insects are the dominant terrestrial arthropods

and are found in all of Earth’s biomes. All insects

have three body segments—a head, thorax, and

abdomen—as well as wings, compound eyes,

and three pairs of legs. Insects grow through

either complete or incomplete metamorphosis.

Breathing through a system of tracheae and spira-

cles helps insects to conserve water.

24.5 Arthropods and HumansArthropods and humans interact in many ways.

Humans and arthropods often compete for the

same resources. Pesticides are a common way in

which humans control arthropod populations.

Arthropods are also vectors for many different

diseases, carrying viruses and bacteria from one

species and infecting another.

Concept Map Use a concept map to summarize the features of arthropods.

Process Diagram Use a process diagram to explain arthropod vectors.

Arthropod feeds

on infected

organismhave have

adapted for used for that must be made of

jointed

appendages

molted

Arthropods

Synthesize Your Notes

752 Unit 8: Animals

Chapter Assessment

24.1 arthropod, p. 730 exoskeleton, p. 730 chitin, p. 730 appendage, p. 730 segmentation, p. 730

24.2 crustacean, p. 735 cephalothorax, p. 735 abdomen, p. 735

carapace, p. 735 mandible, p. 737

24.3 chelicerate, p. 740 arachnid, p. 740 book lung, p. 740 spiracle, p. 741 trachea, p. 741

24.4 incomplete metamorphosis, p. 744

complete metamorphosis, p. 744

pupa, p. 744

24.5 insecticide, p. 747 vector, p. 748

Chapter Vocabulary

Reviewing Vocabulary

Vocabulary Connections

For each pair of words below, write a sentence or two that clearly shows how the terms are connected. For example, for appendage and chelipeds, you could write, “A cheliped is a type of appendage that looks like a large claw.”

1. pupa, metamorphosis

2. cephalothorax, carapace

3. chitin, exoskeleton

4. mandible, chelicerae

Keep It Short

Write a short, precise phrase that defines each vocabulary term below. For example, a short phrase to describe exoskeleton could be “hard, outer covering.”

5. segmentation

6. chelicerae

7. book lung

8. metamorphosis

9. spiracle

Word Origins

10. The term appendage comes from the Latin word appendere, which means “to cause to hang (from something).” Explain how this meaning relates to what an appendage is.

11. The term carapace is a French word that means “tortoise shell.” Explain how this meaning relates to what a carapace is.

Reviewing MAIN IDEAS

12. Each of the thousands of known arthropod species can be placed into one of five groups. What features do arthropods in all five groups have in common?

13. Your skeleton is inside your body, and it grows along with you. How does an arthropod’s exoskeleton differ in both its location and its response to growth?

14. Explain how scientists can determine relationships between ancient arthropods that lived 500 million years ago and arthropods of today.

15. Crustaceans, such as crabs and shrimp, are found in nearly all aquatic food chains. Describe the significance of crustaceans to these ecosystems.

16. Arachnids were some of the first land animals. How did the development of methods for conserving water allow them to colonize land?

17. What important function do arachnids serve in an ecosystem?

18. What features set insects apart from other arthropods?

19. What stages does a butterfly go through during metamorphosis?

20. Insects evolved from ancestors that lived in water. What adaptations did insects develop that allowed them to live on land?

21. Insects and humans interact in many ways. Give two ways in which insects are beneficial to humans, and two ways in which they are harmful to human societies.


CAPTURE EFFICIENCY AND CARAPACE LENGTH

Critical Thinking

22. Classify You turn over a rock and several long, thin animals with many sets of legs scurry away into the leaves. To what group of arthropods do they likely belong? What do they likely eat?

23. Apply If you walk along a rocky shoreline, you will likely see many barnacles attached to rocks. But barnacles can also be found attached to large whales that never swim up on the shore. How do these barnacles get onto whales?

24. Compare and Contrast The tracheae of insects branch out throughout the body. Humans can only breathe through their mouth or nose. What are the advantages and disadvantages of having tracheae instead of a mouth or nose?

25. Apply Termites are a pest species that feed on the cellulose found in wood. They live in large colonies and can destroy houses over the course of a few years if left untreated. What are some ways in which a homeowner might eliminate a termite colony?

26. Infer The world’s largest spider is the goliath bird-eating spider of South America, which can be as large as a dinner plate. What anatomical features may prevent spiders from growing any larger than this?

Interpreting Visuals

Use the diagram to answer the next two questions.

27. Apply What type of metamorphosis is illustrated in the diagram?

28. Identify Name and describe each phase of meta-morphosis shown above. What adaptive advantage does this type of metamorphosis give this species?

Analyzing DataUse the data below to answer the next three questions.

Crab nets have to be designed to capture crabs of varying lengths. The data below show the efficiency of crab nets at capturing crabs of varying carapace length.

29. Interpret What is the relationship between capturing efficiency and carapace length?

30. Interpret Does the scatterplot have any outliers? If so, what explanation might explain the outlier?

Connecting CONCEPTS

31. Write Science Fiction Create your own species of arthropod. Imagine that you are sampling arthropod species 1 million years into the future. Arthropods are very different than they are today. Draw and label the appendages and other features of this new species. Write a brief description of the species, and include your hypothesis on its ancestors, what environmental pressures selected for its features, and describe its habitat, food, and lifestyle.

32. Compare and Contrast Insect predators such as the potter wasp hunt numerous species of arthropods. In what ways are the hunting styles of arachnids and wasps similar and different?

754 Unit 8: Animals

For more test practice,go to ClassZone.com.


1.

Set Up for Daphnia Experiment

Daphnia Treatment

Group A flashes of light from above

Group B flashes of light from below

Group C no light

Two students observed how a species of daphnia,a microscopic crustacean, responds to light.They divided their daphnia into three groupswith each receiving a different light treatment.Based on their experimental design shownabove, which group served as a control?

A Group A

B Group B

C Group C

D no control group in this experiment

2. In one chain of a soil food web, dead plantmaterial is decomposed by fungi. The fungi areeaten by tiny insects called springtails, whichare in turn eaten by centipedes. At each link inthis chain, about 90 percent of the energy

A stays with the primary producer.

B is stored in body structures of the consumer.

C is lost to the environment as heat.

D is transferred to the decomposers.

3. Sowbugs and pillbugs are terrestrial crustaceansthat feed primarily on decaying organic matter.They are important to the stability ofecosystems in part due to their role as

A producers.

B predators.

C decomposers.

D autotrophs.

4. Walking sticks are insects that look like parts ofa plant. They are eaten by birds, lizards, andother predators. Over many generations,natural selection has most likely favoredindividual walking sticks that

A cannot be easily seen by predators.

B look the most frightening to predators.

C stand out the most to predators.

D do not have any predators.

Remember that natural selection favors individuals with adaptations that allow them to survive and reproduce.

THINK THROUGH THE QUESTION

5. Yellow fever is caused by a virus that istransmitted by mosquitoes. Proof of yellowfever vaccination is required before enteringcertain countries. This vaccination containssmall doses of the virus that

A kill any mosquitoes that bite the person.

B allow the person to build up an immunity.

C mutate in the person’s bloodstream.

D kill all viruses in the person’s bloodstream.

6.

Trilobite Abundance in the Fossil Record

Millions of Years Ago (MYA) Fossil Record540 MYA trilobite fossils appear

500 MYA peak of trilobitediversity

450 MYA trilobite diversity drops

300 MYA trilobites uncommon

245 MYA to Present no trilobites

Scientists have generated the above data basedon hundreds of thousands of trilobite fossilsthat have been identified and dated in the fossilrecord. What conclusion can be drawn fromthese data?

A Scientists must look longer for trilobite fossils.

B Trilobite fossils began to decay 300 MYA.

C Trilobite speciation peaked about 300 MYA.

D Trilobites went extinct about 245 MYA.

Set Up for Daphnia Experiment

Trilobite Abundance in the Fossil Record

24 chapter a closer look at arthropods - weebly

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